The present invention is directed to a device for imaging printing plates using an array of n laser diodes.
For some time now, devices and methods have been known, which make it possible to image a printing plate, whether it be a flat or curved surface, through exposure to laser radiation. Devices and methods of this kind are used, in particular, in so-called CtP systems, computer-to-plate, or direct imaging print units or printing presses for manufacturing offset printing forms.
At the present time, printing plates are primarily imaged by laser diode systems. Their inherent system properties prevent them from reaching the physical limits of the beam quality. In particular, their low beam quality limits their depth of focus, so that an autofocusing system is needed at high resolutions. Two different approaches are currently used for multibeam imaging, i.e., for simultaneously exposing a plurality of image points on various media, such as printing plates, films, data carriers or the like. On the one hand, the radiation from individual laser diodes or an array of laser diodes can be directly applied via optical elements, such as lenses, mirrors or fibers, to the medium to be imaged. On the other hand, the radiation from a laser light source, typically laser diode bars, can be projected via diverse optical elements onto an array of n modulators. For the most part, these are electrooptical or acoustooptic modulators. By selectively driving the n modulators, one can select individual beams from the entire radiation and modulate their power. The selected, power-modulated beams are supplied via further optical elements to the medium to be imaged.
European Patent Application No. 0 878 773 A2 describes an optical system for imaging an array of light sources, in particular an individually addressable array of laser substantially greater than their emitter height. The emission region is typically about 1 micrometer high and 60 micrometers wide. The optical system is composed of a system of non-anamorphotic imaging lenses and of a cylinder lens, which is placed between the array and the imaging lens system and images the laser radiation onto the scanning surface. This surface usually does not lie in the foci of the laser beams, so that a widening of the short dimensions of the imaged emission surface occurs.
U.S. Pat. No. 5,521,748 describes a system for exposing image data using an individual laser or an array of diodes and a light modulator. The light transmitted by the laser or the array is imaged onto a modulator having a row of light-modulating elements of the reflectance or transmittance type. Once selection and power modulation are carried out, the radiation is imaged onto a surface having light-sensitive material, forming individual image points. To place image points of this kind on a complete, two-dimensional surface, a relative motion of the image points to the light-sensitive material is provided. In the interplay resulting from generation of the individual points and the relative motion, the desired image data are then exposed on the two-dimensional surface. The relative motion between the light beams emanating from the light modulator and the light-sensitive material can be effected on a cylindrical configuration such that lines are exposed in a meander shape along the axis of symmetry of the cylinder, or such that lines run around the cylinder in a helical form.
U.S. Pat. No. 5,691,759 discusses a multi-beam laser light source, which produces raster scan lines on a medium using the so-called interleaving raster scan line method. The interleaving raster scan line method is distinguished by the following properties. A laser light source emits radiation, from which n image points are produced using modulated power by employing suitable imaging optics and modulation. These n image points are arranged in a row, and the distance between two adjacent points is (n+1)p, p being the distance between the dots. Provision is made between the medium and the image points for a relative motion in both directions, spanning the surface of the medium. Once n points are imaged, the medium is displaced relatively to the image points with a translational component that is perpendicular to the direction defined by the axis of the image points, so that n points can again be exposed at another location of the medium. In this manner, so-called scan lines of image points are formed, initially at a distance of (n+1)p, which are produced by laser radiation, whose power is modulated in dependence upon the image information. Upon completion of a scan having a translational component in the perpendicular direction, a displacement by the distance (nxc3x97p) follows in parallel to the direction defined by the axis of the n image points. The n image points are then shifted again with a translational component that is perpendicular to the direction defined by the axis of the image pixels on the surface, forming further scan lines. Thus, each raster scan line is separated from its immediate neighbor by the pitch distance p between the dots. Using a plurality of optical beams from a laser light source, an overlapping of the scan lines ensues (interleaving raster scan line method).
An enhanced interleaving raster scan line method for a multibeam laser light source is described in European Patent Application No. 0 947 950 A2. In the case of n image points having a pitch distance p of the dots, each of whose adjacent image points are separated by the distance (qxc3x97n+1)p, q being a natural number, an incremental distance of nxc3x97p results by which the medium must be moved between the marking of two scan lines. An overlapping (interleaving) of the scan lines is thereby achieved, in other words, the new scan lines are written between the old scan lines. By properly selecting the displacement in parallel to the axis defined by the image points, by the distance nxc3x97p, an imaging is then possible, without one location, where image information is to be scanned, being repeatedly exposed to one image point of a laser. What distinguishes the described method is that adjacent image points of the laser diodes are spaced further apart, in each case, than the width of the displacement by which the medium is moved between the old and new scan lines.
Various disadvantages are associated with each of the known devices. The radiation emitted by broad array laser diodes, laser diode bars, and laser diode stacks exhibits a low beam quality, as quantified by the diffraction index M2. Even with correction, the attainable depth of focus is only suited for imaging at a low resolution, typically 1,270 dpi. Therefore, to produce very small dots, for example resolutions of about 2,540 dpi, an autofocusing system is necessary, which requires a complex mechanical and electrical design. If the light source and modulator are provided separately, there is an increased requirement for optical, electronic and mechanical components, as well as for substantial overall space. Many components need to be adjusted, and the service life can be clearly limited. The temperature management of the components turns out to be just as problematic. Only a limited, minimal physical size is possible when a device for imaging printing plates is assembled from discrete components. The described interleaving raster scan line method is not suited for compact laser light sources, since the distance between adjacent image points must always be one unit p greater than the number of beams, so that one must revert to scanning methods in which image points are set densely together.
An object of the present invention is to provide a device for imaging printing plates using an array of n laser diodes, whose emitted light exhibits a good beam quality and which renders possible a compact design. An additional or alternate object of the present invention is to provide an improved interleaving raster scan line method.
The present invention provides a device for imaging printing plates using an array (10) of n laser diodes which are imaged onto n image points (110), so that one laser diode (12) is allocated to each i-th point having i from {1, . . . , n}, the n image points (110) being separated by a spatial interval of adjacent points l, and a pitch distance p of the dots being provided. The laser diodes (12) are individually drivable single stripe laser diodes.
The present invention also provides a method, i.e., a so-called interleaving raster scan line method, for imaging printing plates by generating raster points using an array of n laser light sources, which are imaged using an imaging optics onto n image points, which are arranged on a line, the n image points being separated by a spatial interval of adjacent points l, comprising the following method steps:
simultaneous generation of n image points on the printing plate by a number of laser light sources;
generation of a relative motion between the image points and printing plate;
displacement of the image points with a translation component perpendicular to the axis defined by the line of the image points by a first specific amount;
displacement of the n image points in the direction defined by the n image points by a second specific amount; and
iteration of the displacements in question, wherein the amount of the second specific displacement is greater than the spatial interval l of adjacent image points.
In accordance with the present invention, the device for imaging printing plates includes an array of n single stripe laser diodes. Each single stripe laser diode can be driven individually. The n laser beams can preferably be imaged onto the medium using light-transmitting means, such as lenses, mirrors, optical fibers or the like. The n image points produced with the assistance of imaging optics are advantageously disposed on a line and have a spatial interval l between adjacent points. It is generally only necessary, however, that the n image points projected onto a predefined line in the surface of the printing plate have a constant spatial interval l. A relative motion takes place between the medium and the image points in both directions, spanned by the surface of the medium. In addition to the motion, which, in order to displace the image points with a translational component perpendicular to the direction defined by the line of the n image points or by the predefined line, on which the projected n image points exhibit a constant spatial interval l, a displacement takes place in parallel to the direction defined by line of the n image points or by the predefined line, on which the projected n image points exhibit a constant spatial interval l. The amount of this displacement is advantageously greater than or equal to the spatial interval l between the n image points. Raster scan lines are produced which exhibit a pitch distance p between the dots, pitch distance p between the dots being smaller than spatial interval l between the image points.
One preferred specific embodiment provides that the power supply to the array of the laser diodes be regulated. A suitable detector element advantageously checks for proper functioning, and, as the case may be, for potential malfunctioning of a single stripe laser diode, either on the outcoupling side of the laser diode or, however, at another cavity mirror. In this context, the detector element can be both a detector row, as well as an individual detector, which scans the individual single stripe laser diodes.
One derives a number of advantages from the use of an array of n single stripe laser diodes, which can be individually driven, and from the application of the corresponding interleaving raster scan line method to image printing plates. An excellent beam quality is achieved by using single stripe laser diodes. Typically, the value of diffraction index M2 is slightly higher than one. In a compact design, a high level of integration can be achieved: radiation source, modulation, and control can be combined in one component. The result is fewer optical components and, therefore, less need for adjustment of sensitive components. The service life of the component is essentially limited only by the service life of the laser. The compact, modular design makes the system scalable. A high-performance stability is assured by a rapid control. The high level of integration provides for a simpler temperature management, since it is only necessary to cool this one component. Due to the low diffraction index M2, a maximum possible depth of focus is achieved when focusing.